This invention concerns cellular highly crosslinked, three dimensional ethylenic polymer resins having amic acid and/or cyclic imide linkages including (but not limited to) those having polymeric structure illustrated by the formula: ##STR1## wherein R is a divalent organic radical having x valence bonds on different carbon atoms, A is a tetravalent organic radical having a first pair of valences on vicinal carbon atoms attached to carbonyl groups and a second pair of valences on vicinal carbon atoms forming part of the polymer chain and arising by addition polymerization of a C.dbd.C ethylenic double bond in the monomeric form of A, x is a small number at least 2, and n is a large number. Such resin has carbon-to-carbon ethylenic polymer chains and multiple imide crosslinkages. A polymerized N,N'-bis-maleimide of a diamine is an example of such resin where x is 2 and A is ##STR2## in Formula I. Amic acid resins have the group: ##STR3## in place of at least some of the imide groups: ##STR4## in Formula I.
Resins having structure like that of Formula I where x is 2 (i.e., bis-imide polymers) are known in non-cellular form, but foams of such resins have not heretofore been known. For example, such non-cellular, crosslinked resins from N,N'-bis-maleimides are described in U.S. Pat. Nos. 3,380,964 (F. Grundschober et al.), 3,406,148 (J. Sambeth et al.), 3,533,996 (F. Grundschober et al.), and 3,890,272 (G. F. D'Alelio).
The "polyimide foams" that are known in the art have resin structures different from that of Formula I and include those wherein the imide groups are recurring linkages in the main polymer chain such as those obtained by reaction of a dianhydride of a tetracarboxylic acid, e.g., pyromellitic dianhydride, and a diamine as described in U.S. Pat. Nos. 3,249,561 (W. R. Hendrix), 3,310,506 (L. W. Amborski et al.), 3,483,144 (E. Lavin et al.), and 3,554,939 (E. Lavin et al.). Foams having structures similar to those just mentioned are also made by reaction of a tetracarboxylic acid or dianhydride with a polyisocyanate as described in 3,300,420 (H. E. Frey), 3,479,305 (S. T. Kus et al.), 3,489,696 (G. W. Miller), 3,562,189 (W. J. Farrissey, Jr. et al.), 3,620,987 (A. M. McLaughlin et al.), 3,644,234 (R. L. Grieve), and 3,772,216 (R. W. Rosser et al.). Another kind of foam from resins having "imide" linkages is described in German Offenlegungsschrift No. 25-17-06 by F. Zumstein et al. None of these prior art foam resins has resin polymeric structure like that of Formula I.
It would be desirable if there were method and means to make cellular highly crosslinked ethylenic polymer resins having the general structure of Formula I wherein the foam-forming and resin finishing step could be carried out using a storage-stable foamable resin intermediate.
This invention provides method and means for making such storage-stable foamable resin intermediate and for making therefrom cellular highly crosslinked three dimensional ethylenic polymer resins having amic acid and/or cyclic imide linkages, including (but not limited to) those having structure generally shown in Formula I. It also makes such storage-stable foamable resin intermediate in granular or powdered solid form. Other advantages of the invention are evident in the descriptions that follow.